The present invention relates generally to spatial multiplexing in a multi-user multiple-input, multiple output (MU-MIMO) communication system and, more particularly, to a methods and apparatus for demodulating received signals in a MU-MIMO communication system.
In recent years, there has been much interest in multiple input, multiple output (MIMO) systems for enhancing data rates in mobile communication systems. MIMO systems employ multiple antennas at the transmitter and receiver to transmit and receive information. By exploiting the spatial dimension of the communication channel between the transmitting terminal and the receiving terminal, MIMO communication systems can simultaneously transmit multiple data streams from a transmitting terminal to one or more receiving terminals over the same carrier frequency. Thus, MIMO communication systems achieve higher spectral efficiency and higher data rates without increasing bandwidth.
One transmission scheme for MIMO systems that is receiving significant attention is spatial multiplexing. In a spatial multiplexing transmitter, several independently encoded data streams are precoded and transmitted with appropriate weighting from each one of the transmit antennas with appropriate weighting so that the link throughput is maximized. The Long Term Evolution (LTE) standard supports several spatial multiplexing modes, including one for multi-user MIMO (MU-MIMO). In this mode, multiple data streams are transmitted by the transmitting terminal to different receiving terminals. Thus, from the perspective of a given receiving terminal, there will be at least one data stream, referred to as the desired data stream, intended for that receiving terminal and one or more other data streams, referred to herein as interfering data streams, intended for other receiving terminals.
Typically, the set of precoding weights, i.e. precoder, for the desired data stream is signaled to the receiving terminal so that it will know the effective channel associated with the desired data stream. However, the precoders for the interfering data streams may not be signaled, so the receiving terminal typically employs Maximum Ratio Combining (MRC) to demodulate the received signal. Because the interference attributable to the data streams intended for other user terminals is correlated, MRC combining is suboptimal.